This invention relates to an electric cell unit comprising chargeable electric cells such as lithium ion electric cells and a protection circuit for protecting the electric cells by detecting overcharge or overdischarge of each of the electric cells and, in particular, to a detection circuit unit to be used in the protection circuit for detecting the overcharge and over discharge of one electric cell.
Among conventional chargeable electric cells, a lithium ion electric cell is particularly weak against overcharge and overdischarge. Therefore, the lithium ion electric cell is essential to be accompanied with the protection circuit for detecting an overcharged condition and an overdischarged condition to stop a charging operation and a discharging operation, respectively. In detail, the lithium ion electric cell (hereinafter called an electric cell) is made to connection to a load or a charging power supply or a charging device through a protection switch and is also connected to the detection circuit which comprises an overcharge detection circuit and an overdischarge detection circuit connected in parallel to each other.
The overcharge detection circuit is for comparing an output voltage or a terminal voltage of the electric cell with a first reference voltage. When the terminal voltage is higher than the first reference voltage, it is determined or judged that the electric cell is overcharged. At that time, the protection switch is turned off so that the electric cell is electrically disconnected from the charging device to stop the charging operation.
On the other hand, the overdischarge detection circuit is for comparing the terminal voltage of the electric cell with a second reference voltage which is lower than the first reference voltage. When the terminal voltage is lower than the second reference voltage, it is determined or judged that the electric cell is overdischarged. At that time, the protection switch is turned off so that the electric cell is electrically disconnected from the load to stop the discharging operation.
In each of the overcharge detection circuit and the overdischarge detection circuit, comparison of the voltages is carried out by the use of a voltage comparing circuit. The voltage comparing circuit comprises a reference voltage generating zener diode connected to a constant-current source, bleeder resistors, and a comparator. A detection accuracy by the voltage comparing circuit is determined by an accuracy of the reference voltage, a resistance ratio of the bleeder resistors, and an offset voltage of the comparator.
In a known electric cell unit of the type having a plurality of electric cells, a plurality of detection circuits are provided for individually monitoring the output voltages of the electric cells, respectively. When overdischarge or overcharge is detected for any one of the electric cells, the protection switch is turned off. Those detection circuits have been formed on a single chip so that the protection circuit can be small-sized, easy in the tramming operation during the production of the detection circuit on the chap, and made with a common reference voltage source circuit for a plurality of detection circuits. Therefore, a two-cell type electric cell unit was provided with a protection circuit exclusively usable for the two-cell type, while a three-cell type electric cell unit was provided with a protection circuit exclusively usable for the three-cell type, for instance the protection circuit for the two-cell type electric cell unit cannot bemused for a protection circuit in the three-cell type electric cell unit. On the contrary, the protection circuit for the three-cell type electric cell unit cannot be used in the two-cell type electric cell unit.
Accordingly, it was required for various electric cell units having different numbers of electric cells that different protection circuits were produced and stored for production and maintenance of the electric cell units.